This invention relates to a pyroelectric infra-red detector comprising a substantially planar pyroelectric element onto which infra-red radiation to be detected is directed, and which is supported in spaced and substantially parallel relationship over the surface of a body acting as a heat sink, the element and the body being thermally coupled. The invention relates also to a method of making such a detector.
Pyroelectric detectors are used for a variety of different purposes, for example in remote switching, and in movement sensing systems such as intruder alarms. Directing infra-red radiation to be detected onto the pyroelectric element causes a change in the temperature of the element. This temperature change generates electrical charges at the opposing electrodes which charges can be made to flow as a current through a comparatively low external impedance or to produce a voltage across the element if the external impedance is comparatively high. If the pyroelectric element is arranged as a capacitor in an amplifying circuit, the resultant current or the voltage developed can be detected. Since the pyroelectric charge is produced only while the temperature of the element is changing, it is necessary for the temperature to be varied continuously to obtain a continuous electrical signal. This may be done by chopping the incident radiation at a uniform frequency, the element being exposed to radiation at a reference temperature whilst the radiation of interest is cut off.
One application of pyroelectric detectors is in infra-red spectrometry. For such purposes, the pyroelectric element of the detector commonly comprises triglycine sulphate (TGS) as the pyroelectric material because of its suited sensitivity. Such pyroelectric detectors are preferred to other types of thermal detectors in view of their superior performance in the 10 Hz to 5kHz frequency region at which IR spectrometers are operated and their ability to respond to a wide range of wavelengths without the need for forced cooling. The pyroelectric element, which needs to be very thin for optimal performance, is mounted in an hermetically sealed envelope provided with a window overlying the element and through which radiation of the wavelength range of interest is directed onto the element.
IR spectrometers can be of the dispersive type or the Fourier transform type. These two types differ in that in the latter type the whole of the wavelength range of the incident infra-red radiation source is directed onto the detector whereas only narrow wavelength intervals are used in the former type. Consequently, in a Fourier transform spectrometer the energy received by the detector, which may be as much as 100 mW or more, could over a period of time cause the temperature of the pyroelectric element to rise gradually bearing in mind the thinness of the element. This in turn can lead to a change in output as the responsivity of the pyroelectric element, and particularly, but not exclusively, a TGS element, is not independent of operating temperature. Thermally coupling the pyroelectric element to a body acting as a heatsink is intended to minimize this problem. The combination of element and body has a shorter thermal time constant than the element alone. The hermetically-sealed envelope in which the element is mounted typically contains a gas such as dry nitrogen so that a relatively high thermal resistance exists between the element and the surrounding atmosphere. The heatsink body associated with the element has a comparatively high thermal mass, and consequently a high thermal inertia, and is designed to act as a temperature regulator and stabilize the temperature of the pyroelectric element at approximately its optimum operating point. The function of the heatsink body is thus to maintain the temperature of the pyroelectric element substantially constant in operation of the detector so that the responsivity remains much the same over a time period even though the received radiation may be high in energy.
In a known IR detector employing a thin planar TGS element, the element is bonded to a face of the heatsink body in spaced relationship by means of a centrally-disposed blob of thermally-conductive bonding material such as conductive epoxy or solder. A drop of the bonding material is placed on the face of the heatsink body and the element is then placed over the body and pushed towards the body to compress the bonding material until the opposing element and body faces are a predetermined distance apart. In its compressed state the bonding material occupies only a very small proportion of the area of the surface of the element, the remaining surface area being physically spaced from the opposing body face forming a gap between the element and the body extending around the bonding material. This gap is eventually filled by the encapsulant gas, for example nitrogen which provides a degree of thermal coupling between the element and the body. Some thermal coupling between these parts is also provided by the bonding material.
The area occupied by the bonding material is minimized in order to reduce problems arising from the physical properties of the element. Some pyroelectric materials such as TGS have anisotropic thermal expansion characteristics and if elements of these materials were to be bonded to a substrate by means of bonding material applied over too great a proportion of their surface area, this being typically around 7 square millimeters for a TGS detector element, they would tend to crack when subjected to thermal cycling.
To achieve the desired close thermal coupling, the spacing between the facing surfaces of the element and the heatsink body determined by the blob of bonding material in its compressed state needs to be very small, and would normally be less than 5 micrometers for acceptable performance of the detector when nitrogen gas is used.
Whilst such detectors have been found to work reasonably well, problems are experienced in both manufacture and use. Because the bonding material is located at only a relatively small central area of the element, it is difficult to achieve reasonable parallelism between the pyroelectric element and the face of the heatsink body. This can give rise in use to variations in temperature of the element over its area, for example at opposite edges, leading to non-uniform response. Also, the very small spacing necessary between the element and the body can lead to a high loss rate in manufacture as any dust particles or the like present in the gap may cause the element to crack when pressure is applied to the element during bonding to the body.